The present invention relates to a spherical annular seal member used in a spherical pipe joint for an automobile exhaust pipe.
As a spherical annular seal member used in a spherical pipe joint for an automobile exhaust pipe, one disclosed in, for example, Japanese Patent Application Laid-Open No. 76759/1979 is known. The seal member disclosed in this publication is heat resistant, excels in affinity with a mating member, and has remarkably improved impact strength, but has a drawback in that the seal member often generates abnormal noise when it undergoes friction under dry frictional conditions. The drawback of this seal member is conceivably attributable to, among others, the fact that there is a large difference between the coefficient of static friction and the coefficient of dynamic friction of a heat-resistant material, such as expanded graphite, for forming the seal member, and to the fact that the frictional resistance of the seal member constituted by this heat-resistant material exhibits negative resistance with respect to the sliding speed.
Therefore, to overcome the above-described drawback, a seal member disclosed in Japanese Patent Application Laid-Open No. 123362/1994 (corresponding to U.S. Pat. No. 5,499,825) was proposed. This seal member excels in the sealing characteristic without generating abnormal frictional noise in friction with a mating member, and satisfies the performance required of a seal member.
However, with respect to this proposed seal member as well, a new problem has been presented which is attributable to, among others, the improvement in recent years of the performance of automobile engines. That is, the conventional seal members are unable to satisfy the conditions of use in the light of heat resistance, owing to an increase in the exhaust-gas temperature due to the improved performance of the automobile engines, or owing to an increase in the exhaust-gas temperature attributable to the fact that the spherical pipe joint is located closer to the engine side in a case where the spherical pipe joint is disposed in the vicinity of a manifold, i.e., an outlet of the exhaust gases, for the purpose of improving the noise, vibration and harshness (NVH) characteristics of an automobile. Thus, there has been a compelling need for improvement of the heat resistance of the seal member itself.